1. Field of the Invention
The present invention relates to a laser beam deflection apparatus for deflecting a picture drawing laser beam by utilizing a polygon mirror.
2. Description of the Prior Art
In order to understand the present invention more clearly, let us first explain a previously-proposed projection type video display apparatus using a laser beam which is described in Japanese Laid-Open Utility Model Gazette No. 56-152456 or The Journal of the Institute of Television Engineers of Japan, Vol. 29, No. 2, 1975. FIG. 1 shows an overall arrangement of such a previously-proposed video display apparatus of the projection type.
Referring to FIG. 1, there are provided laser light sources 1a and 1b which might be a semiconductor laser, a gas laser or the like. A red laser beam from the laser light source 1a is introduced into an optical modulator 2a. A green laser beam and a blue laser beam from the laser light source 1b are introduced into a dichroic mirror 3a, in which they are separated to provide a green laser beam and a blue laser beam. The green laser beam is introduced into an optical modulator 2b, whereas the blue laser beam is introduced through a reflection prism 4a to an optical modulator 2c. Modulation signals corresponding to three primary color signals, provided as a video signal of an image to be displayed, are supplied to the optical modulators 2a, 2b and 2c, respectively. The optical modulators 2a, 2b and 2c modulate in intensity (luminous intensity) the red, green and blue laser beams on the basis of the modulation signals. The laser beams from the optical modulators 2a, 2b and 2c are supplied to lenses 5a, 5b and 5c for adjusting the diameters of the laser beams, respectively. The blue laser beam from the lens 5c is introduced into a dichroic mirror 3b through a reflection prism 4b, and the green laser beam from the lens 5b is introduced into the other mirror face of the dichroic mirror 3b, whereby the blue and green laser beams are mixed. The laser beam thus mixed is introduced into a dichroic mirror 3c, and the red laser beam from the lens 5a is introduced into the other mirror face of the dichroic mirror 3c, whereby a mixed laser beam of three primary colors is obtained. The modulated, mixed laser beam is supplied to a reflection portion 11 of a rotating polygon mirror 10.
In the polygon mirror 10, the reflecting portion 11 is formed by arranging a plurality of flat mirrors with an equal spacing in an annular-fashion. This annular-shaped reflecting portion 11 is rotated at high speed by some suitable drive means such as a motor or the like. The flat mirrors forming the reflecting portion 11 of the polygon mirror 10 may be a regular icosipentahedron thereby to deflect a laser beam incident on each flat mirror. FIGS. 2A and 2B illustrate the deflected states of the laser beam.
As, for example, shown in FIG. 2A, when a laser beam l.sub.in becomes incident on an end portion of a flat mirror 11.sub.1 of the reflecting portion 11 by the rotation of the reflecting portion 11, a laser beam l.sub.out is reflected towards the lower portion of FIG. 2A. When the reflecting portion 11 is rotated, the angle at which the laser beam l.sub.in becomes incident on the flat mirror 11.sub.1 is gradually changed, thus changing the outgoing direction of the reflected laser beam l.sub.out. As shown in FIG. 2B, when the reflecting portion 11 is rotated by an angle .THETA..sub.1 to cause the laser beam l.sub.in to become incident on the other end portion of the flat mirror 11.sub.1, a reflected laser beam l.sub.out ' is oriented towards the upper portion of FIG. 2B. In that event, an angle .THETA..sub.2 formed by the laser beams l.sub.out and l.sub.out ' becomes the deflection angle of the flat mirror 11.sub.1, i.e. the angle through which the laser beam is scanned as the reflecting portion 11 is rotated. This constitutes one "deflection cycle " of the rotating reflection portion 11. Other flat mirrors of the reflecting portion 11 deflect the laser beams by a similar deflection angle so that, when the reflecting portion 11 is composed of 25 flat mirrors, the laser beam is deflected through the angle .THETA..sub.2 25 times per revolution of the reflecting portion 11.
Turning back to FIG. 1, the laser beam reflected by the polygon mirror 10 is introduced through a projection lens 6 to a galvano mirror 7 which is controlled in rotation by a drive source 7a. When the galvano mirror 7 is rotated during a predetermined interval by the drive source 7a, the laser beam from the polygon mirror 10 is deflected through a predetermined deflection angle over a predetermined interval. In that case, the deflection direction by the polygon mirror 10 and the deflection direction by the galvano mirror 7 are selected to be at right angles to each other. The deflection by the polygon mirror 10 corresponds to the horizontal deflection in the television receiver, whereas the deflection by the galvano mirror 7 corresponds to the vertical deflection in the television receiver.
A laser beam reflected by the galvano mirror 7 is reflected by a reflection mirror 8 and is made incident on the rear surface of a flat screen 9. Upon use, the viewer can see an image, formed by the scanning laser beams, from the front surface side of the screen 9.
Horizontal and vertical scanning cycles of the video signal for forming the modulation signals supplied to the optical modulators 2a, 2b and 2c are synchronized with the deflection cycle of the polygon mirror 10 and the deflection cycle of the galvano mirror 7, whereby an image based on the video signal is displayed on the screen 9 by the raster-scanning of the laser beams. Therefore, an image of one field-time is displayed on the screen 9 during one field period of the video signal. Hence, the display apparatus of FIG. 1 is operated as a projection-type video display apparatus.
The operation of the thus arranged video display apparatus will be described next.
When a video signal, for example, of 1125 horizontal scanning lines, is displayed, the polygon mirror 10 having 25 flat mirrors must be rotated at 81000 r.p.m., which needs a special drive motor and a special bearing for effecting such high speed rotation. If the polygon mirror 10 is rotated at such high speed, then the flat mirrors of the polygon mirror will be moved at a high speed beyond the velocity of sound, which is not desirable from a video display apparatus safety standpoint.
In order to solve the above-described problems, it is proposed to decrease the rotation speed of the polygon mirror by increasing the number of the flat mirrors formed on the reflecting portion of the polygon mirror. This proposal is not advantageous in practice because of the following reasons:
If the rotating portion of the polygon mirror is increased in diameter, then a greater centrifugal force is applied to the reflecting portion of the polygon mirror, thereby producing an elastic strain on the mirror surface. As a result, the raster-scanning of the laser beam is disturbed and also a large torque motor is unavoidably needed, which is not practical. Further, if the number of the flat mirrors is increased without increasing the diameter of the rotating portion of the polygon mirror, then the area of one flat mirror will be decreased so that the deflection angle becomes very small from a laser beam and beam spot relationship standpoint, which is also not advantageous in practice.
An example of the aforementioned defect will be described hereinunder.
In the flat mirror formed on each plane of the polygon mirror having pentacontane reflection faces and a diameter of 40 cm, the width of each flat mirror becomes 2.5 cm. When a laser beam having a spot diameter of 1 mm becomes incident on the polygon mirror, if the laser beam becomes incident on both planes at the boundary portion of the two flat mirrors, then a so-called eclipse will occur. This eclipse makes respective end portions of each plane become invalid by 1 mm each, making 40% of each plane invalid. Therefore, only 60% of the plane can be effectively utilized, and hence the deflection angle becomes very small, which is not advantageous in practice.
Furthermore, if the beam diameter of the laser beam is reduced, then the resolution of the image to be displayed will be degraded. Accordingly, it is not possible to reduce the invalid period by reducing the beam diameter of the laser beam.